Convert rich text content in html format to plain text

It is strongly recommended that the client implements this conversion locally. If it must be implemented through the server API interface, call the following method: ```javascript WXAPI.stringsToPlainText(content, len) ``` + content html format rich text content + len returns the number of characters. If it exceeds the number, it will be truncated. If empty, all characters will be returned. ```json { "code": 0, "data": "test", "msg": "success" } ``` + data The plain text content returned